High-performance membrane electrode unit and the use thereof in fuel cells

ABSTRACT

The present invention relates to a membrane electrode unit comprising a polymer membrane doped with a mineral acid as well as two electrodes, characterized in that the polymer membrane comprises at least one polymer with at least one nitrogen atom and at least one electrode comprises a catalyst which is formed from at least one precious metal and at least one metal less precious according to the electrochemical series.

RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. § 371)of PCT/EP2005/0001761, filed Feb. 20, 2005, which claims benefit ofGerman Application No. 10 2004 008 628.1, filed Feb. 21, 2004.

BACKGROUND OF THE INVENTION

The present invention relates to high-performance membrane electrodeunits, which can particularly be used in so-called polymer electrolytemembrane fuel cells.

A fuel cell usually contains an electrolyte and two electrodes separatedby the electrolyte, in which one of the two electrodes is supplied witha fuel, such as hydrogen gas or a mixture of methanol and water, and theother electrode is supplied with an oxidant, such as oxygen gas or air.In the process, chemical energy generated by the resulting fueloxidation is directly converted into electric power.

One requirement of the electrolyte is that it is permeable to hydrogenions, i.e. protons, but not to the fuels mentioned above.

Typically, a fuel cell comprises several individual cells, so-calledMEUs (membrane electrode unit), each of which contains an electrolyteand two electrodes separated by the electrolyte.

As electrolyte for the fuel cell, solids, such as polymer electrolytemembranes, or liquids, such as phosphoric acid, are applied. Polymerelectrolyte membranes have recently attracted interest as electrolytesfor fuel cells.

Polymer electrolyte membranes with complexes of alkaline polymers andstrong acids are known from WO96/13872, for example. To produce these,an alkaline polymer, for example polybenzimidazole, is treated with astrong acid, such as phosphoric acid.

Furthermore, fuel cells whose membrane comprises inorganic supportmaterials, such as for example glass-fibre fabrics or glass-fibre veils,which are saturated with phosphoric acid, are also known, for examplefrom U.S. Pat. No. 4,017,664.

In the alkaline polymer membranes known in the prior art, the mineralacid (mostly concentrated phosphoric acid) used—to achieve the requiredproton conductivity—is usually added following the forming of thepolyazole film. In doing so, the polymer serves as a support for theelectrolyte consisting of the highly concentrated phosphoric acid. Inthe process, the polymer membrane fulfils further essential functions,particularly, it has to exhibit a high mechanical stability and serve asa separator for the two fuels mentioned at the outset.

An essential advantage of such a membrane doped with phosphoric acid isthe fact that a fuel cell in which such a polymer electrolyte membraneis employed can be operated at temperatures above 100° C. without thehumidification of the fuels otherwise necessary. This is due to thecharacteristic of the phosphoric acid to be able to transport theprotons without additional water via the so-called Grotthus mechanism(K.-D. Kreuer, Chem. Mater. 1996, 8, 610-641).

Further advantages for the fuel cell system are achieved through thepossibility of operation at temperatures above 100° C. On the one hand,the sensitivity of the Pt catalyst to gas impurities, in particular CO,is reduced substantially. CO is formed as a by-product in the reformingof hydrogen-rich gas from carbon-containing compounds, such as e.g.natural gas, methanol or benzine, or also as an intermediate product inthe direct oxidation of methanol. Typically, the CO content of the fuelhas to be lower than 100 ppm at temperatures <100° C. However, attemperatures in the range of from 150-200°, 10,000 ppm CO or more canalso be tolerated (N. J. Bjerrum et. al., Journal of AppliedElectrochemistry, 2001, 31, 773-779). This results in substantialsimplifications of the upstream reforming process and thereforereductions of the cost of the entire fuel cell system.

The output of a membrane electrode unit produced with such membranes isdescribed in WO 01/18894 and in Electrochimica Acta, Volume 41, 1996,193-197 and amounts to less than 0.2 A/cm² with a platinum loading of0.5 mg/cm² (anode) and 2 mg/cm² (cathode) and a voltage of 0.6 V. Whenusing air instead of oxygen, this value drops to less than 0.1 A/cm².

A big advantage of fuel cells is the fact that, in the electrochemicalreaction, the energy of the fuel is directly converted into electricpower and heat. In the process, water is formed at the cathode as areaction product. Heat is also produced in the electrochemical reactionas a by-product. In applications in which only the power for theoperation of electric motors is utilised, such as e.g. in automotiveapplications, or as a versatile replacement of battery systems, part ofthe heat generated in the reaction has to be dissipated to preventoverheating of the system. Additional energy-consuming devices whichfurther reduce the total electric efficiency of the fuel cell are thenneeded for cooling. In stationary applications, such as for thecentralised or decentralised generation of electricity and heat, theheat can be used efficiently by existing technologies, such as e.g. heatexchangers. In doing so, high temperatures are aimed for to increase theefficiency. If the operating temperature is higher than 100° C. and thetemperature difference between the ambient temperature and the operatingtemperature is high, it will be possible to cool the fuel cell systemmore efficiently, for example using smaller cooling surfaces anddispensing with additional devices, in comparison to fuel cells whichhave to be operated at less than 100° C. due to the humidification ofthe membrane.

SUMMARY OF THE INVENTION

The membrane electrode units set forth above already show a goodproperty profile, however, the capability, for example the currentdensity at high voltages, of known membrane electrode units has to beimproved further.

A further object was to provide a membrane electrode unit which exhibitsa high capability, in particular a high current density or a highcurrent density at a high voltage, respectively, over a wide range oftemperatures.

Additionally, the membrane electrode unit according to the invention hasto display high durability, in particular a long service life at thehigh power densities demanded.

An ongoing object in all membrane electrode units is to lower thequantities of catalysts to minimise the production costs without therebyreducing the capability significantly. Advantageously, it should bepossible to operate the membrane electrode unit with little gas flowand/or with low excess pressure achieving high power density.

Therefore, the present invention has the object to provide a novelmembrane electrode unit which solves the objects set forth above.

The object of the present invention is a membrane electrode unitcomprising

-   A) at least one polymer membrane which includes at least one polymer    with at least one nitrogen atom, the polymer membrane including at    least one mineral acid,-   B) at least two electrodes,    characterized in that at least one electrode comprises a catalyst    containing-   i. at least one precious metal of the platinum group, in particular    Pt, Pd, Ir, Rh, Os, Ru, and/or at least one precious metal Au and/or    Ag-   ii. at least one metal less precious according to the    electrochemical series as the metal mentioned in (i.), in particular    selected from the group of Fe, Co, Ni, Cr, Mn, Zr, Ti, Ga, V

A DETAILED DESCRIPTION OF THE INVENTION

The membrane electrode unit according to the invention provides highcurrent intensities without the voltage being lowered too much. Themembrane electrode units further exhibit good durability at high currentintensities.

A fundamental aspect and technical advantage of the membrane electrodeunit according to the invention is that the excellent capability isobtained with a low concentration of catalytically active substances,such as for example platinum, ruthenium or palladium, heretofore notachieved.

The polymers which comprise at least one nitrogen atom are formed in thepolymer membrane included in the membrane electrode unit according tothe invention. These polymers are also called alkaline polymers. Thealkalinity of the polymer can also be defined via the molar ratio ofnitrogen atoms to carbon atoms. The scope of the present inventionencompasses in particular such polymers whose molar ratio of nitrogenatoms to carbon atoms is in the range of from 1:1 to 1:100, preferablyin the range of from 1:2 to 1:20. This ratio can be determined byelemental analysis.

Alkaline polymers, in particular polymers with at least one nitrogenatom, are known in professional circles. In general, polymers with onenitrogen atom in the backbone and/or in the side chain can be used.

The polymers with one nitrogen atom include, for example,polyphosphazenes, polyimines, polyisocyanides, polyetherimine,polyaniline, polyamides, polyhydrazides, polyurethanes, polyimides,polyazoles and/or polyazines.

Preferably, the polymer membranes comprise polymers with at least onenitrogen atom used in a repeating unit. In this connection, it is alsopossible to use copolymers which, in addition to repeating units withone nitrogen atom, also comprise repeating units without a nitrogenatom.

According to a particular aspect of the present invention, alkalinepolymers with at least one nitrogen atom are used. The term alkaline isknown in professional circles in which this is to be understood inparticular as Lewis and Brønstedt alkalinity.

The repeating unit in the alkaline polymer preferably contains anaromatic ring with at least one nitrogen atom. The aromatic ring ispreferably a five- to six-membered ring with one to three nitrogen atomswhich can be fused to another ring, in particular another aromatic ring.

Polymers based on polyazole generally contain recurring azole units ofthe general formula (I) and/or (II) and/or (III) and/or (IV) and/or (V)and/or (VI) and/or (VII) and/or (VIII) and/or (IX) and/or (X) and/or(XI) and/or (XII) and/or (XIII) and/or (XIV) and/or (XV) and/or (XVI)and/or (XVII) and/or (XVIII) and/or (XIX) and/or (XX) and/or (XXI)and/or (XXII).

wherein

-   Ar are identical or different and represent a tetracovalent aromatic    or heteroaromatic group which can be mononuclear or polynuclear,-   Ar¹ are identical or different and represent a bicovalent aromatic    or heteroaromatic group which can be mononuclear or polynuclear,-   Ar² are identical or different and represent a bicovalent or    tricovalent aromatic or heteroaromatic group which can be    mononuclear or polynuclear,-   Ar³ are identical or different and represent a tricovalent aromatic    or heteroaromatic group which can be mononuclear or polynuclear,-   Ar⁴ are identical or different and represent a tricovalent aromatic    or heteroaromatic group which can be mononuclear or polynuclear,-   Ar⁵ are identical or different and represent a tetracovalent    aromatic or heteroaromatic group which can be mononuclear or    polynuclear,-   Ar⁶ are identical or different and represent a bicovalent aromatic    or heteroaromatic group which can be mononuclear or polynuclear,-   Ar⁷ are identical or different and represent a bicovalent aromatic    or heteroaromatic group which can be mononuclear or polynuclear,-   Ar⁸ are identical or different and represent a tricovalent aromatic    or heteroaromatic group which can be mononuclear or polynuclear,-   Ar⁹ are identical or different and represent a bicovalent or    tricovalent or tetracovalent aromatic or heteroaromatic group which    can be mononuclear or polynuclear,-   Ar¹⁰ are identical or different and represent a bicovalent or    tricovalent aromatic or heteroaromatic group which can be    mononuclear or polynuclear,-   Ar¹¹ are identical or different and represent a bicovalent aromatic    or heteroaromatic group which can be mononuclear or polynuclear,-   X are identical or different and represent oxygen, sulphur or an    amino group which carries a hydrogen atom, a group having 1-20    carbon atoms, preferably a branched or unbranched alkyl or alkoxy    group, or an aryl group as a further radical,-   R are identical or different and represent hydrogen, an alkyl group    and an aromatic group, with the proviso that R in the formula (XX)    is not hydrogen, and-   n, m are each an integer greater than or equal to 10, preferably    greater or equal to 100.

Aromatic or heteroaromatic groups preferred according to the inventionare derived from benzene, naphthalene, biphenyl, diphenyl ether,diphenylmethane, diphenyldimethylmethane, bisphenone, diphenylsulphone,thiophene, furan, pyrrole, thiazole, oxazole, imidazole, isothiazole,isoxazole, pyrazole, 1,3,4-oxadiazole, 2,5-diphenyl-1,3,4-oxadiazole,1,3,4-thiadiazole, 1,3,4-triazole, 2,5-diphenyl-1,3,4-triazole,1,2,5-triphenyl-1,3,4-triazole, 1,2,4-oxadiazole, 1,2,4-thiadiazole,1,2,4-triazole, 1,2,3-triazole, 1,2,3,4-tetrazole, benzo[b]thiophene,benzo[b]furan, indole, benzo[c]thiophene, benzo[c]furan, isoindole,benzoxazole, benzothiazole, benzimidazole, benzisoxazole,benzisothiazole, benzopyrazole, benzothiadiazole, benzotriazole,dibenzofuran, dibenzothiophene, carbazole, pyridine, bipyridine,pyrazine, pyrazole, pyrimidine, pyridazine, 1,3,5-triazine,1,2,4-triazine, 1,2,4,5-triazine, tetrazine, quinoline, isoquinoline,quinoxaline, quinazoline, cinnoline, 1,8-naphthyridine,1,5-naphthyridine, 1,6-naphthyridine, 1,7-naphthyridine, phthalazine,pyridopyrimidine, purine, pteridine or quinolizine, 4H-quinolizine,diphenyl ether, anthracene, benzopyrrole, benzooxathiadiazole,benzooxadiazole, benzopyridine, benzopyrazine, benzopyrazidine,benzopyrimidine, benzotriazine, indolizine, pyridopyridine,imidazopyrimidine, pyrazinopyrimidine, carbazole, aziridine, phenazine,benzoquinoline, phenoxazine, phenothiazine, acridizine, benzopteridine,phenanthroline and phenanthrene which optionally also can besubstituted.

In this case, Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ can have anysubstitution pattern, in the case of phenylene, for example, Ar¹, Ar⁴,Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ can be ortho-, meta- and para-phenylene.Particularly preferred groups are derived from benzene and biphenylenewhich optionally also can be substituted.

Preferred alkyl groups are short-chain alkyl groups having 1 to 4 carbonatoms, e.g. methyl, ethyl, n- or i-propyl and t-butyl groups.

Preferred aromatic groups are phenyl or naphthyl groups. The alkylgroups and the aromatic groups can be substituted.

Preferred substituents are halogen atoms, e.g. fluorine, amino groups,hydroxy groups or short-chain alkyl groups, e.g. methyl or ethyl groups.

Polyazoles having recurring units of the formula (I) are preferredwherein the radicals X within one recurring unit are identical.

The polyazoles can in principle also have different recurring unitswherein their radicals X are different, for example. It is preferable,however, that a recurring unit has only identical radicals X.

Further preferred polyazole polymers are polyimidazoles,polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines,polythiadiazoles, poly(pyridines), poly(pyrimidines) andpoly(tetrazapyrenes).

In another embodiment of the present invention, the polymer containingrecurring azole units is a copolymer or a blend which contains at leasttwo units of the formulae (I) to (XXII) which differ from one another.The polymers can be in the form of block copolymers (diblock, triblock),random copolymers, periodic copolymers and/or alternating polymers.

In a particularly preferred embodiment of the present invention, thepolymer containing recurring azole units is a polyazole which onlycontains units of the formulae (I) and/or (II).

The number of recurring azole units in the polymer is preferably aninteger greater than or equal to 10. Particularly preferred polymerscontain at least 100 recurring azole units.

Within the scope of the present invention, polymers containing recurringbenzimidazole units are preferred. Some examples of the most appropriatepolymers containing recurring benzimidazole units are represented by thefollowing formulae:

wherein n and m are each an integer greater than or equal to 10,preferably greater than or equal to 100.

Further preferred polyazole polymers are polyimidazoles,polybenzimidazole ether ketone, polybenzothiazoles, polybenzoxazoles,polytriazoles, polyoxadiazoles, polythiadiazoles, polypyrazoles,polyquinoxalines, poly(pyridines), poly(pyrimidines) andpoly(tetrazapyrenes).

Preferred polyazoles are characterized by a high molecular weight. Thisapplies in particular to the polybenzimidazoles. Measured as theintrinsic viscosity, this is in the range of from 0.3 to 10 dl/g,preferably 1 to 5 dl/g.

Celazole from the company Celanese is particularly preferred. Theproperties of polymer film and polymer membrane can be improved byscreening the starting polymer, as described in German patentapplication No. 10129458.1.

Very particular preference is given to using para-polybenzimidazoles inthe production of the polymer electrolyte membranes. In this connection,the polybenzimidazoles comprise in particular six-membered aromaticgroups which are linked at the 1,4 position. Particular preference isgiven to using poly-[2,2′-(p-phenylene)-5,5′-bisbenzimidazole].

The polymer film used for the doping and based on alkaline polymers cancomprise still more additives in the form of fillers and/or auxiliaries.Additionally, the polymer film can feature further modifications, forexample by cross-linking, as described in German patent application No.1010752.8 or in WO 00/44816. In a preferred embodiment, the polymer filmused for the doping and consisting of an alkaline polymer and at leastone blend component additionally contains a cross-linking agent, asdescribed in German patent application No. 10140147.7. An essentialadvantage of such a system is the fact that higher doping levels andtherefore a greater conductivity with sufficient mechanical stability ofthe membrane can be achieved.

In addition to the above-mentioned alkaline polymers, a blend of one ormore alkaline polymers with another polymer can be used. In this case,the function of the blend component is essentially to improve themechanical properties and reduce the cost of material. Here,polyethersulphone is a preferred blend component, as described in Germanpatent application No. 10052242.4.

The preferred polymers which can be employed as the blend componentinclude, amongst others, polyolefines, such as poly(chloroprene),polyacetylene, polyphenylene, poly(p-xylylene), polyarylmethylene,polyarmethylene, polystyrene, polymethylstyrene, polyvinyl alcohol,polyvinyl acetate, polyvinyl ether, polyvinyl amine, poly(N-vinylacetamide), polyvinyl imidazole, polyvinyl carbazole, polyvinylpyrrolidone, polyvinyl pyridine, polyvinyl chloride, polyvinylidenechloride, polytetrafluoroethylene, polyhexafluoropropylene, copolymersof PTFE with hexafluoropropylene, with perfluoropropylvinyl ether, withtrifluoronitrosomethane, with sulphonyl fluoride vinyl ether, withcarbalkoxyperfluoroalkoxyvinyl ether, polychlorotrifluoroethylene,polyvinyl fluoride, polyvinylidene fluoride, polyacrolein,polyacrylamide, polyacrylonitrile, polycyanoacrylates,polymethacrylimide, cycloolefinic copolymers, in particular ofnorbornenes;

polymers having C—O bonds in the backbone, for example polyacetal,polyoxymethylene, polyether, polypropylene oxide, polyepichlorohydrin,polytetrahydrofuran, polyphenylene oxide, polyether ketone, polyester,in particular polyhydroxyacetic acid, polyethyleneterephthalate,polybutyleneterephthalate, polyhydroxybenzoate, polyhydroxypropionicacid, polypivalolacton, polycaprolacton, polymalonic acid,polycarbonate;polymer with C—S bonds in the backbone, for example polysulphide ether,polyphenylenesulphide, polyethersulphone;polymer with C—N bonds in the backbone, for example polyimines,polyisocyanides, polyetherimine, polyaniline, polyamides,polyhydrazides, polyurethanes, polyimides, polyazoles, polyazines;liquid crystalline polymers, in particular Vectra, as well as inorganicpolymers, such as polysilanes, polycarbosilanes, polysiloxanes,polysilicic acid, polysilicates, silicons, polyphosphazenes andpolythiazyl.

For the application in fuel cells with a long-term service temperatureabove 100° C., such blend polymers that have a glass transitiontemperature or Vicat softening point VST/A/50 of at least 100° C.,preferably at least 150° C. and very particularly preferably at least180° C., are preferred. In this connection, polysulphones with a Vicatsoftening point VST/A/50 of from 180° C. to 230° C. are preferred.

The preferred polymers include polysulphones, in particular polysulphonehaving aromatic groups in the backbone. According to a particular aspectof the present invention, preferred polysulphones and polyethersulphoneshave a melt volume rate MVR 300/21.6 of less than or equal to 40 cm³/10min, in particular less than or equal to 30 cm³/10 min and particularlypreferably less than or equal to 20 cm³/10 min, measured in accordancewith ISO 1133.

According to a particular aspect, the polymer membrane can comprise atleast one polymer with aromatic sulphonic acid groups. Aromaticsulphonic acid groups are groups in which the sulphonic acid groups(—SO₃H) are bound covalently to an aromatic or heteroaromatic group. Thearomatic group can be part of the backbone of the polymer or part of aside group wherein polymers having aromatic groups in the backbone arepreferred. In many cases, the sulphonic acid groups can also be employedin the form of their salts. Furthermore, derivatives, for exampleesters, in particular methyl or ethyl esters, or halides of thesulphonic acids can be used which are converted to the sulphonic acidduring operation of the membrane.

Preferred aromatic or heteroaromatic groups are derived from benzene,naphthalene, biphenyl, diphenyl ether, diphenylmethane,diphenyldimethylmethane, bisphenone, diphenylsulphone, thiophene, furan,pyrrole, thiazole, oxazole, imidazole, isothiazole, isoxazole, pyrazole,1,3,4-oxadiazole, 2,5-diphenyl-1,3,4-oxadiazole, 1,3,4-thiadiazole,1,3,4-triazole, 2,5-diphenyl-1,3,4-triazole,1,2,5-triphenyl-1,3,4-triazole, 1,2,4-oxadiazole, 1,2,4-thiadiazole,1,2,4-triazole, 1,2,3-triazole, 1,2,3,4-tetrazole, benzo[b]thiophene,benzo[b]furan, indole, benzo[c]thiophene, benzo[c]furan, isoindole,benzoxazole, benzothiazole, benzimidazole, benzisoxazole,benzisothiazole, benzopyrazole, benzothiadiazole, benzotriazole,dibenzofuran, dibenzothiophene, carbazole, pyridine, bipyridine,pyrazine, pyrazole, pyrimidine, pyridazine, 1,3,5-triazine,1,2,4-triazine, 1,2,4,5-triazine, tetrazine, quinoline, isoquinoline,quinoxaline, quinazoline, cinnoline, 1,8-naphthyridine,1,5-naphthyridine, 1,6-naphthyridine, 1,7-naphthyridine, phthalazine,pyridopyrimidine, purine, pteridine or quinolizine, 4H-quinolizine,diphenyl ether, anthracene, benzopyrrole, benzooxathiadiazole,benzooxadiazole, benzopyridine, benzopyrazine, benzopyrazidine,benzopyrimidine, benzotriazine, indolizine, pyridopyridine,imidazopyrimidine, pyrazinopyrimidine, carbazole, aziridine, phenazine,benzoquinoline, phenoxazine, phenothiazine, acridizine, benzopteridine,phenanthroline and phenanthrene which optionally also can besubstituted. Preferred substituents are halogen atoms, such as e.g.fluorine, amino groups, hydroxy groups or alkyl groups.

In this case, the substitution pattern can be in any form, in the caseof phenylene, for example, it can be ortho-, meta- and para-phenylene.Particularly preferred groups are derived from benzene and biphenylenewhich optionally also can be substituted.

Preferred alkyl groups are short-chain alkyl groups having 1 to 4 carbonatoms, e.g. methyl, ethyl, n- or i-propyl and t-butyl groups.

Preferred aromatic groups are phenyl or naphthyl groups. The alkylgroups and the aromatic groups can be substituted.

The polymers modified with sulphonic acid groups preferably have acontent of sulphonic acid groups in the range of from 0.5 to 3 meq/g,preferably 0.5 to 2 meq/g. This value is determined through theso-called ion exchange capacity (IEC).

To measure the IEC, the sulphonic acid groups are converted to the freeacid. To this end, the polymer is treated in a known way with acid,removing excess acid by washing. Thus, the sulphonated polymer isinitially treated for 2 hours in boiling water. Subsequently, excesswater is dabbed off and the sample is dried at 160° C. in a vacuumdrying cabinet at p<1 mbar for 15 hours. Then, the dry weight of themembrane is determined. The polymer thus dried is then dissolved in DMSOat 80° C. for 1 h. Subsequently, the solution is titrated with 0.1MNaOH. The ion exchange capacity (IEC) is then calculated from theconsumption of acid to reach the equivalence point and from the dryweight.

Polymers with sulphonic acid groups covalently bound to aromatic groupsare known in professional circles. Polymers with aromatic sulphonic acidgroups can, for example, be produced by sulphonation of polymers.Processes for the sulphonation of polymers are described in F. Kucera etal., Polymer Engineering and Science 1988, Vol. 38, No. 5, 783-792. Inthis connection, the sulphonation conditions can be chosen such that alow degree of sulphonation develops (DE-A-19959289).

With regard to polymers having aromatic sulphonic acid groups whosearomatic radicals are part of the side group, particular reference shallbe made to polystyrene derivatives. The document U.S. Pat. No. 6,110,616for instance describes copolymers of butadiene and styrene and theirsubsequent sulphonation for use in fuel cells.

Furthermore, such polymers can also be obtained by polyreactions ofmonomers which comprise acid groups. Thus, perfluorinated polymers asdescribed in U.S. Pat. No. 5,422,411 can be produced by copolymerisationof trifluorostyrene and sulphonyl-modified trifluorostyrene.

According to a particular aspect of the present invention,thermoplastics stable at high temperatures which include sulphonic acidgroups bound to aromatic groups are employed. In general, such polymershave aromatic groups in the backbone. Thus, sulphonated polyetherketones (DE-A-4219077, WO96/01177), sulphonated polysulphones (J. Membr.Sci. 83 (1993), p. 211) or sulphonated polyphenylenesulphide(DE-A-19527435) are preferred.

The polymers set forth above which have sulphonic acid groups bound toaromatic groups can be used individually or as a mixture whereinmixtures having polymers with aromatic groups in the backbone areparticularly preferred.

The molecular weight of the polymers having sulphonic acid groups boundto aromatic groups can vary widely, depending on the type of polymer andits processibility. Preferably, the weight average of the molecularweight M_(w) is in the range of from 5000 to 10,000,000, in particular10,000 to 1,000,000, particularly preferably 15,000 to 50,000. Accordingto a particular aspect of the present invention, polymers with sulphonicacid groups bound to aromatic groups which have a low polydispersityindex M_(w)/M_(n) are used. Preferably, the polydispersity index is inthe range of from 1 to 5, in particular 1 to 4.

To further improve the properties in terms of application technology,the flat material can feature fillers, in particular proton-conductingfillers.

Non-limiting examples of proton-conducting fillers are

-   sulphates, such as CsHSO₄, Fe(SO₄)₂, (NH₄)₃H(SO₄)₂, LiHSO₄, NaHSO₄,    KHSO₄, RbSO₄, LiN₂H₅SO₄, NH₄HSO₄,-   phosphates, such as Zr₃(PO₄)₄, Zr(HPO₄)₂, HZr₂(PO₄)₃,    UO₂PO_(4.3)H₂O, H₈UO₂PO₄, Ce(HPO₄)₂, Ti(HPO₄)₂, KH₂PO₄, NaH₂PO₄,    LiH₂PO₄, NH₄H₂PO₄, CSH₂PO₄, CaHPO₄, MgHPO₄, HSbP₂O₈, HSb₃P₂O₁₄,    H₅Sb₅P₂O₂₀,-   polyacid, such as H₃PW₁₂O₄₀.nH₂O (n=21-29), H₃SiW₁₂O₄₀.nH₂O    (n=21-29), H_(x)WO₃, HSbWO₆, H₃PMo₁₂O₄₀, H₂Sb₄O₁₁, HTaWO₆, HNbO₃,    HTiNbO₅, HTiTaO₅, HSbTeO₆, H₅Ti₄O₉, HSbO₃, H₂MoO₄-   selenites and arsenides, such as (NH₄)₃H(SeO₄)₂, UO₂AsO₄,    (NH₄)₃H(SeO₄)₂, KH₂AsO₄, Cs₃H(SeO₄)₂, Rb₃H(SeO₄)₂,-   phosphides, such as ZrP, TiP, HfP-   oxides, such as Al₂O₃, Sb₂O₅, ThO₂, SnO₂, ZrO₂, MoO₃-   silicates, such as zeolites, zeolites (NH₄+), phyllosilicates,    tectosilicates, H-natrolites, H-mordenites, NH₄-analcines,    NH₄-sodalites, NH₄-gallates, H-montmorillonites-   acids, such as HClO₄, SbF₅-   fillers, such as carbides, in particular SiC, Si₃N₄, fibres, in    particular glass fibres, glass powders and/or polymer fibres,    preferably based on polyazoles.

These additives can be included in the proton-conducting polymermembrane in usual amounts, however, the positive properties of themembrane, such as great conductivity, long service life and highmechanical stability, should not be affected too much by the addition oftoo large amounts of additives. Generally, the membrane comprises notmore than 80% by weight, preferably not more than 50% by weight andparticularly preferably not more than 20% by weight, of additives.

To produce the polymer film, the polymer components are initiallydissolved or suspended, as described in the applications cited above,for example DE No. 10110752.8 or WO 00/44816, and subsequently used forthe production of the polymer films. Furthermore, the polymer films inaccordance with DE No. 10052237.8 can be produced continuously.

Alternatively, the formation of the film can take place in accordancewith the process described in the Japanese application No. Hei10-125560.

In this, the solution is poured into a cylinder with a cylindricalinterior surface and the cylinder is then set into rotation. At the sametime, the solvent is allowed to evaporate via the centrifugal forcecaused by the rotation, thereby forming a cylindrical polymer film onthe interior surface of the cylinder wherein the film has a thicknessthat is largely uniform.

With this process, the alkaline polymer having a uniform matrix can beformed. This process described in the Japanese patent application Hei10-125560 is also part of the present description.

Then, the solvent is removed. This can take place by methods known tothe person skilled in the art, for example by drying.

The film made of alkaline polymer or polymer blend is subsequentlyimpregnated or doped with a strong acid, preferably a mineral acid,wherein the film can be treated as described previously in the Germanpatent application No. 10109829.4. This variant is beneficial to excludeinteractions of the residual solvent with the barrier layer.

To this end, the film comprising at least one polymer with at least onenitrogen atom is immersed in a strong acid such that the film isimpregnated with the strong acid and becomes a proton-conductingmembrane. For this, the preferably alkaline polymer is usually immersedin a highly concentrated strong acid at a temperature of at least 35° C.for a period of time of from several minutes up to several hours.

Mineral acid, in particular phosphoric acid and/or sulphuric acid, isused as the strong acid.

Within the scope of the present description, “phosphoric acid” meanspolyphosphoric acid (H_(n+2)PnO_(3n+1) (n>1), which usually has acontent of at least 83%, calculated as P₂O₅ (by acidimetry), phosphonicacid (H₃PO₃), orthophosphoric acid (H₃PO₄), pyrophosphoric acid(H₄P₂O₇), triphosphoric acid (H₅P₃O₁₀) and metaphosphoric acid. Thephosphoric acid, in particular orthophosphoric acid, preferably has aconcentration of at least 80 percent by weight, particularly preferablya concentration of at least 85 percent by weight, even more preferably aconcentration of at least 87 percent by weight and very particularlypreferably a concentration of at least 89 percent by weight. It is to beunderstood that the reason for this is that the preferably alkalinepolymer can be impregnated with a greater number of molecules of thestrong acid at an increasing concentration of the strong acid.

The obtained polymer electrolyte membrane is proton-conducting. Afterthe doping, the degree of doping should be, expressed as mole of acidper repeating unit, greater than 6, preferably greater than 8 and veryparticularly preferably greater than 9.

Instead of the polymer membranes based on preferably alkaline polymersand produced by classical processes, it is also possible to use thepolyazole-containing polymer membranes, as described in the Germanpatent applications No. 10117686.4, 10144815.5, 10117687.2. Such polymerelectrolyte membranes provided with at least one barrier layer are alsoan object of the present invention.

Polymer membranes can preferably be obtained by a process comprising thesteps of

-   i) preparation of a mixture comprising    -   polyphosphoric acid,    -   at least one polyazole and/or at least one or more compounds        which are suitable for the formation of polyazoles with action        of heat in accordance with step ii),-   ii) heating the mixture obtainable in accordance with step i) under    inert gas to temperatures of up to 400° C.,-   iii) applying a layer using the mixture in accordance with step i)    and/or ii) to a support,-   iv) treatment of the membrane formed in step iii).

To this end, one or more compounds can be added to the mixture inaccordance with step i), which are suitable for the formation ofpolyazoles with action of heat in accordance with step ii).

For this, mixtures are suitable which comprise one or more aromaticand/or heteroaromatic tetraamino compounds and one or more aromaticand/or heteroaromatic carboxylic acids or their derivatives, whichcomprise at least two acid groups per carboxylic acid monomer.Furthermore, one or more aromatic and/or heteroaromaticdiaminocarboxylic acids can be used for the preparation of polyazoles.

The aromatic and heteroaromatic tetraamino compounds include, amongstothers, 3,3′,4,4′-tetraaminobiphenyl, 2,3,5,6-tetraaminopyridine,1,2,4,5-tetraaminobenzene, 3,3′,4,4′-tetraaminodiphenyl sulphone,3,3′,4,4′-tetraaminodiphenyl ether, 3,3′,4,4′-tetraaminobenzophenone,3,3′,4,4′-tetraaminodiphenylmethane and3,3′,4,4′-tetraaminodiphenyldimethylmethane

and their salts, in particular their monohydrochloride, dihydrochloride,trihydrochloride and tetrahydrochloride derivatives. Of these,3,3′,4,4′-tetraaminobiphenyl, 2,3,5,6-tetraaminopyridine and1,2,4,5-tetraaminobenzene are particularly preferred.

Furthermore, the mixture i) can comprise aromatic and/or heteroaromaticcarboxylic acids. These are dicarboxylic acids and tricarboxylic acidsand tetracarboxylic acids or their esters or their anhydrides or theiracid halides, especially their acid halides and/or acid bromides.Preferably, the aromatic dicarboxylic acids are isophthalic acid,terephthalic acid, phthalic acid, 5-hydroxyisophthalic acid,4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid,5-aminoisophthalic acid, 5-N,N-dimethylaminoisophthalic acid,5-N,N-diethylaminoisophthalic acid, 2,5-dihydroxyterephthalic acid,2,6-dihydroxyisophthalic acid, 4,6-dihydroxyisophthalic acid,2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid,3,4-dihydroxyphthalic acid, 3-fluorophthalic acid, 5-fluoroisophthalicacid, 2-fluoroterephthalic acid, tetrafluorophthalic acid,tetrafluoroisophthalic acid, tetrafluoroterephthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, diphenylether-4,4′-dicarboxylic acid, benzophenone-4,4′-dicarboxylic acid,diphenylsulphone-4,4′-dicarboxylic acid, biphenyl-4,4′-dicarboxylicacid, 4-trifluoromethylphthalic acid,2,2-bis-(4-carboxyphenyl)hexafluoropropane, 4,4′-stilbenedicarboxylicacid, 4-carboxycinnamic acid or their C1-C20 alkyl esters or C5-C12 arylesters or their acid anhydrides or their acid chlorides.

Very particular preference is given to using mixtures which comprisedicarboxylic acids, the acid radicals of which are in the para position,such as for example terephthalic acid.

The heteroaromatic carboxylic acids are heteroaromatic dicarboxylicacids and tricarboxylic acids and tetracarboxylic acids or their estersor their anhydrides. Heteroaromatic carboxylic acids are understood tomean aromatic systems which contain at least one nitrogen, oxygen,sulphur or phosphorus atom in the aromatic group. Preferably, these arepyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid,pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid,4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic acid,2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid,2,4,6-pyridinetricarboxylic acid, benzimidazole-5,6-dicarboxylic acidand their C1-C20 alkyl esters or C5-C12 aryl esters or their acidanhydrides or their acid chlorides.

Furthermore, the mixture i) can also comprise aromatic andheteroaromatic diaminocarboxylic acids. These include, amongst others,diaminobenzoic acid, 4-phenoxycarbonyl-3′,4′-diaminodiphenyl ether andtheir monohydrochloride and dihydrochloride derivatives.

The mixture produced in step i) preferably comprises at least 0.5% byweight, in particular 1 to 30% by weight and particularly preferably 2to 15% by weight, of monomers for the preparation of polyazoles.

According to another aspect of the present invention, the mixtureproduced in step i) comprises compounds which are suitable for theformation of polyazoles with action of heat in accordance with step ii),these compounds being obtainable by reaction of one or more aromaticand/or heteroaromatic tetraamino compounds with one or more aromaticand/or heteroaromatic carboxylic acids or their derivatives, whichcontain at least two acid groups per carboxylic acid monomer, or of oneor more aromatic and/or heteroaromatic diaminocarboxylic acids in themelt at temperatures of up to 400° C., in particular up to 350° C.,preferably up to 280° C. The compounds to be used for the preparation ofthese prepolymers were set forth above.

Furthermore, monomers which comprise covalently bound acid groups can beused for the preparation of polyazoles. These include, amongst others,aromatic and heteroaromatic dicarboxylic acids or their derivativeswhich include at least one phosphonic acid group, for example2,5-dicarboxyphenylphosphonic acid, 2,3-dicarboxyphenylphosphonic acid,3,4-dicarboxyphenylphosphonic acid and 3,5-dicarboxyphenylphosphonicacid; aromatic and heteroaromatic dicarboxylic acids or theirderivatives which include at least one sulphonic acid group, inparticular 2,5-dicarboxyphenylsulphonic acid,2,3-dicarboxyphenylsulphonic acid, 3,4-dicarboxyphenylsulphonic acid and3,5-dicarboxyphenylsulphonic acid; aromatic and heteroaromaticdiaminocarboxylic acids which comprise at least one phosphonic acidgroup, for example 2,3-diamino-5-carboxyphenylphosphonic acid,2,3-diamino-6-carboxyphenylphosphonic acid and3,4-diamino-6-carboxyphenylphosphonic acid; aromatic and heteroaromaticdiaminocarboxylic acids which comprise at least one sulphonic acidgroup, for example 2,3-diamino-5-carboxyphenylsulphonic acid,2,3-diamino-6-carboxyphenylsulphonic acid and3,4-diamino-6-carboxyphenylsulphonic acid.

A polyazole membrane produced in accordance with the process set forthabove can include the optional components set forth above. These includein particular blend polymers and fillers. Blend polymers can, amongstother things, be dissolved, dispersed or suspended in the mixtureobtained in accordance with step i) and/or step ii). In this connection,the weight ratio of polyazole to polymer (B) is preferably in the rangeof from 0.1 to 50, preferably from 0.2 to 20, particularly preferablyfrom 1 to 10; however, this should not constitute a limitation. If thepolyazole is only formed in step ii), it is possible to arrive at theweight ratio by calculations based on the weight of the monomers for theformation of the polyazole in which the compounds released in thecondensation, for example water, are to be taken into account.

To further improve the properties in terms of application technology,fillers, in particular proton-conducting fillers, and additional acidscan additionally be added to the membrane. The addition can be performedin step i), step ii) and/or step iii), for example. Furthermore, theseadditives can also be added after the polymerisation in accordance withstep iv), if they are in the form of a liquid. These additives weredescribed above.

The polyphosphoric acid used in step i) is a customary polyphosphoricacid as is available, for example, from Riedel-de Haen. Thepolyphosphoric acids H_(n+2)P_(n)O_(3n+1) (n>1) usually have aconcentration of at least 83%, calculated as P₂O₅ (by acidimetry).Instead of a solution of the monomers, it is also possible to produce adispersion/suspension.

The mixture obtained in step i) is, in accordance with step ii), heatedto a temperature of up to 400° C., in particular 350° C., preferably upto 280° C., in particular 100° C. to 250° C. and particularly preferablyin the range of from 200° C. to 250° C. For this, an inert gas, forexample nitrogen, or a noble gas, such as neon, argon, is used.

The mixture produced in step i) and/or step ii) can additionally alsocontain organic solvents. These can affect the processibility in apositive way. For example, the rheology of the solution can be improvedsuch that this can be more easily extruded or applied with a doctorblade.

The formation of the flat structure in accordance with step iii) isperformed by means of measures known per se (pouring, spraying,application with a doctor blade, extrusion) which are known from theprior art of polymer film production. Every support that is consideredas inert under the conditions is suitable as a support. These supportsinclude in particular films made of polyethylene terephthalate (PET),polytetrafluoroethylene (PTFE), polyhexafluoropropylene, copolymers ofPTFE with hexafluoropropylene, polyimides, polyphenylenesulphides (PPS)and polypropylene (PP). Furthermore, the formation of the membrane canalso be performed directly on the electrode provided with a barrierlayer.

The thickness of the flat structure in accordance with step iii) ispreferably between 10 and 4000 μm, preferably between 15 and 3500 μm, inparticular between 20 and 3000 μm, particularly preferably between 30and 1500 μm and very particularly preferably between 50 and 1200 μm.

The treatment of the membrane in step iv) is performed in particular attemperatures in the range of from 0° C. to 150° C., preferably attemperatures between 10° C. and 120° C., in particular between roomtemperature (20° C.) and 90° C., in the presence of moisture or waterand/or steam. The treatment is preferably performed at normal pressure,but can also be carried out with action of pressure. It is essentialthat the treatment takes place in the presence of sufficient moisturewhereby the polyphosphoric acid present contributes to thesolidification of the membrane by means of partial hydrolysis withformation of low molecular weight polyphosphoric acid and/or phosphoricacid.

The partial hydrolysis of the polyphosphoric acid in step iv) leads to asolidification of the membrane and to a reduction in the layer thicknessand the formation of a membrane. The solidified membrane generally has athickness of between 15 and 3000 μm, preferably 20 and 2000 μm, inparticular between 20 and 1500 μm.

The upper temperature limit for the treatment in accordance with stepiv) is typically 150° C. With extremely short action of moisture, forexample from overheated steam, this steam can also be hotter than 150°C. The duration of the treatment is substantial for the upper limit ofthe temperature.

The partial hydrolysis (step iv)) can also take place in climaticchambers wherein the hydrolysis can be specifically controlled withdefined moisture action. In this connection, the moisture can bespecifically set via the temperature or saturation of the surroundingarea in contact with it, for example gases such as air, nitrogen, carbondioxide or other suitable gases, or steam. The duration of the treatmentdepends on the parameters chosen as aforesaid.

Furthermore, the duration of the treatment depends on the thickness ofthe membrane.

Typically, the duration of the treatment amounts to a few seconds tominutes, for example with action of overheated steam, or up to wholedays, for example in the open air at room temperature and lower relativehumidity. Preferably, the duration of the treatment is 10 seconds to 300hours, in particular 1 minute to 200 hours.

If the partial hydrolysis is performed at room temperature (20° C.) withambient air having a relative humidity of 40-80%, the duration of thetreatment is 1 to 200 hours.

The membrane obtained in accordance with step iv) can be formed in sucha way that it is self-supporting, i.e. it can be detached from thesupport without any damage and then directly processed further, ifapplicable.

The treatment in accordance with step iv) leads to curing of thecoating. If the membrane is directly formed on the electrode, thetreatment in accordance with step iv) is performed until the coatingexhibits a sufficient hardness so that it can be pressed to form amembrane electrode unit. A sufficient hardness is given when a membranetreated accordingly is self-supporting. In many cases, however, a lesserhardness is sufficient. The hardness determined in accordance with DIN50539 (microhardness measurement) is generally at least 1 mN/mm²,preferably at least 5 mN/mm² and very particularly preferably at least50 mN/mm²; however, this should not constitute a limitation.

The concentration and the amount of phosphoric acid and therefore theconductivity of the polymer membrane can be set via the degree ofhydrolysis, i.e. the duration, temperature and ambient humidity. Theconcentration of the phosphoric acid is given as mole of acid per moleof repeating unit of the polymer. Within the scope of the presentinvention, a concentration (mole of phosphoric acid, based on arepeating unit of the formula (III), i.e. polybenzimidazole) between 13and 80, in particular between 15 and 80 is preferred. Only with verymuch difficulty or not at all is it possible to obtain such high degreesof doping (concentrations) by doping polyazoles with commerciallyavailable orthophosphoric acid.

Preferred polymer membranes show high proton conductivity. This is atleast 0.1 S/cm, preferably at least 0.11 S/cm, in particular at least0.12 S/cm at temperatures of 120° C.

If the membranes comprise polymers with sulphonic acid groups, themembranes also show high conductivity at a temperature of 70° C. Theconductivity depends, amongst other things, on the content of sulphonicacid groups of the membrane. The higher this proportion, the better isthe conductivity at low temperatures. In this connection, a membraneaccording to the invention can be humidified at low temperatures. Tothis end, for example, the compound employed as the energy source, forexample hydrogen, can be provided with a proportion of water. In manycases, however, the water formed by the reaction is sufficient toachieve a humidification.

The specific conductivity is measured by means of impedancy spectroscopyin a 4-pole arrangement in potentiostatic mode and using platinumelectrodes (wire, diameter of 0.25 mm). The distance between thecurrent-collecting electrodes is 2 cm. The spectrum obtained isevaluated using a simple model comprised of a parallel arrangement of anohmic resistance and a capacitor. The cross-section of the specimen ofthe membrane doped with phosphoric acid is measured immediately beforemounting the specimen. To measure the temperature dependency, themeasurement cell is brought to the desired temperature in an oven andregulated using a Pt-100 thermocouple arranged in the immediate vicinityof the specimen. After the temperature has been reached, the specimen iskept at this temperature for 10 minutes before beginning themeasurement.

A membrane electrode unit according to the invention comprises, inaddition to the polymer membrane, at least two electrodes which are eachin contact with the membrane.

Generally, the electrode comprises a gas diffusion layer. The gasdiffusion layer in general exhibits electron conductivity. Flat,electrically conductive and acid-resistant structures are commonly usedfor this. These include, for example, carbon-fibre paper, graphitisedcarbon-fibre paper, carbon-fibre fabric, graphitised carbon-fibre fabricand/or flat structures which were rendered conductive by addition ofcarbon black.

Furthermore, the electrode includes at least one catalyst layer whichcomprises

-   i. at least one precious metal of the platinum group, in particular    Pt, Pd, Ir, Rh, Os, Ru, and/or at least one precious metal Au and/or    Ag-   ii. at least one metal less precious according to the    electrochemical series as the metal mentioned in (i.), in particular    selected from the group of Fe, Co, Ni, Cr, Mn, Zr, Ti, Ga, V.

Preferably, the catalyst is formed in the form of an alloy of the metals(i) and (ii). In addition to the alloy, further catalytically activesubstances, in particular precious metals of the platinum group, i.e.Pt, Pd, Ir, Rh, Os, Ru, or also the precious metals Au and Ag, can beused. Furthermore, the oxides of the above-mentioned precious metalsand/or non-precious metals can also be employed.

The catalytically active particles which comprise the above-mentionedsubstances can be employed as metal powder, so-called black preciousmetal, in particular platinum and/or platinum alloys. Such particlesgenerally have a size in the range of from 5 nm to 200 nm, preferably inthe range of from 7 nm to 100 nm.

Furthermore, the metals can also be employed on a support material.Preferably, this support comprises carbon which particularly can be usedin the form of carbon black, graphite or graphitised carbon black.Furthermore, electrically conductive metal oxides, such as for example,SnO_(x), TiO_(x), or phosphates, such as e.g. FePO_(x), NbPO_(x),Zr_(y)(PO_(x))_(z), can be used as support material. In this connection,the indices x, y and z designate the oxygen or metal content of theindividual compounds which can lie within a known range as thetransition metals can be in different oxidation stages.

The content of these metal particles on a support, based on the totalweight of the bond of metal and support, is generally in the range offrom 1 to 80% by weight, preferably 5 to 60% by weight and particularlypreferably 10 to 50% by weight; however, this should not constitute alimitation. The particle size of the support, in particular the size ofthe carbon particles, is preferably in the range of from 20 to 100 nm,in particular 30 to 60 nm. The size of the metal particles presentthereon is preferably in the range of from 1 to 20 nm, in particular 1to 10 nm and particularly preferably 2 to 6 nm.

The sizes of the different particles represent mean values and can bedetermined via transmission electron microscopy or X-ray powderdiffractometry.

The catalyst layer has a thickness in the range of from 0.1 to 50 μm.

The membrane electrode unit according to the invention has a catalystloading of between 0.01 and 20 g/m², preferably 0.1 and 10 g/m² (sum ofall precious metals), based on the surface area of the polymer membrane.

The weight ratio of the precious metals of the platinum group or of Auand/or Ag to the metals less precious according to the electrochemicalseries is between 1:100 and 100:1.

The catalytically active particles set forth above can generally beobtained commercially.

The preparation of the catalysts is described in US392512,JP06007679A940118 or EP450849A911009, for example.

The catalyst can, amongst other things, be applied to the gas diffusionlayer. Subsequently, the gas diffusion layer provided with a catalystlayer can be bonded with a polymer membrane to obtain a membraneelectrode unit according to the invention.

Furthermore, the polymer membrane can be provided with a catalyst layerincluding

-   i. at least one precious metal of the platinum group, in particular    Pt, Pd, Ir, Rh, Os, Ru, and/or at least one precious metal Au and/or    Ag-   ii. at least one metal less precious according to the    electrochemical series as the metal mentioned in (i.), in particular    selected from the group of Fe, Co, Ni, Cr, Mn, Zr, Ti, Ga, V.

This membrane can then be bonded with a gas diffusion layer. In thisconnection, it is also possible to use a gas diffusion layer providedwith a catalyst layer. In general, however, a gas diffusion layer whichdoes not comprise any catalyst is sufficient.

To apply at least one catalyst layer to a polymer membrane, severalmethods can be employed. For example, a support can be used in step iii)which is provided with a coating containing a catalyst to provide thelayer formed in step iii) or iv) with a catalyst layer.

In this connection, the membrane can be provided with a catalyst layeron one side or both sides. If the membrane is provided with a catalystlayer only on one side, the opposite side of the membrane has to bepressed together with an electrode which comprises a catalyst layer. Ifboth sides of the membrane are to be provided with a catalyst layer, thefollowing methods can also be applied in combination to achieve anoptimal result.

In the membrane electrode unit according to the invention, the catalystscontained in the electrode or the catalyst layer adjacent to the gasdiffusion layer at the side of the cathode and anode differ. In aparticularly preferred embodiment of the invention, at least the cathodeside comprises a catalyst containing

-   i. at least one precious metal of the platinum group, in particular    Pt, Pd, Ir, Rh, Os, Ru, and/or at least one precious metal Au and/or    Ag-   ii. at least one metal less precious according to the    electrochemical series as the metal mentioned in (i.), in particular    selected from the group of Fe, Co, Ni, Cr, Mn, Zr, Ti, Ga, V.

According to the invention, the catalyst layer can be applied by aprocess in which a catalyst suspension is employed. Additionally,powders which comprise the catalyst can be used.

The catalyst suspension contains a catalytically active substance. Thesewere described above.

Furthermore, the catalyst suspension can contain customary additives.These include, amongst others, fluoropolymers, such as e.g.polytetrafluoroethylene (PTFE), thickeners, in particular water-solublepolymers, such as e.g. cellulose derivatives, polyvinyl alcohol,polyethylene glycol, and surface-active substances.

The surface-active substances include in particular ionic surfactants,for example salts of fatty acids, in particular sodium laurate,potassium oleate; and alkylsulphonic acids, salts of alkylsulphonicacids, in particular sodium perfluorohexanesulphonate, lithiumperfluorohexanesulphonate, ammonium perfluorohexanesulphonate,perfluorohexanesulphonic acid, potassium nonafluorobutanesulphonate, aswell as non-ionic surfactants, in particular ethoxylated fatty alcoholsand polyethylene glycols.

Furthermore, the catalyst suspension can comprise components that areliquid at room temperature. These include, amongst others, organicsolvents which can be polar or non-polar, phosphoric acid,polyphosphoric acid and/or water. The catalyst suspension preferablycontains 1 to 99% by weight, in particular 10 to 80% by weight, ofliquid components.

The polar organic solvents include in particular alcohols, such asethanol, propanol and/or butanol.

The organic, non-polar solvents include, amongst others, known thinningagents for thin layers, such as the thinning agent for thin layers 8470from the company DuPont which comprises oils of turpentine.

Fluoropolymers, in particular tetrafluoroethylene polymers, representparticularly preferred additives. According to a particular embodimentof the present invention, the weight ratio of fluoropolymer to catalystmaterial comprising at least one precious metal and optionally one ormore support materials is greater than 0.1, this ratio preferably lyingwithin the range of from 0.2 to 0.6.

The catalyst suspension can be applied to the membrane by customaryprocesses. Depending on the viscosity of the suspension which can alsobe in the form of a paste, several methods are known by which thesuspension can be applied. Processes for coating films, fabrics,textiles and/or paper, in particular spraying methods and printingprocesses, such as for example screen and silk screen printingprocesses, inkjet printing processes, application with rollers, inparticular anilox rollers, application with a slit nozzle andapplication with a doctor blade, are suitable. The corresponding processand the viscosity of the catalyst suspension depend on the hardness ofthe membrane.

The viscosity can be controlled via the solids content, especially theproportion of catalytically active particles, and the proportion ofadditives. The viscosity to be adjusted depends on the method ofapplication of the catalyst suspension, the optimal values and thedetermination thereof being familiar to the person skilled in the art.

Depending on the hardness of the membrane, an improvement of the bond ofcatalyst and membrane can be effected by heating and/or pressing.Additionally, the bond between membrane and catalyst improves through atreatment in accordance with step iv).

Furthermore, the application of a catalyst layer can be carried out atthe same time as the treatment of the membrane in accordance with stepiv) until the membrane is self-supporting. This can take place, forexample, by applying a catalyst suspension containing water to the flatstructure in accordance with step iii). Here, the suspension can besprayed onto the flat surface in accordance with step iii) in the formof fine droplets. In addition to water, the suspension can also containfurther solvents and/or thickeners. Depending on the water content, thecuring of the membrane is performed in accordance with step iv).Accordingly, the water content can be within wide ranges. The watercontent preferably is in the range of from 0.1 to 99, in particular 1 to95% by weight, based on the catalyst suspension.

According to a particular aspect of the present invention, the catalystlayer is applied by a powder process. In this connection, a catalystpowder is used which can contain additional additives which wereexemplified above.

To apply the catalyst powder, spraying processes and screeningprocesses, amongst others, can be employed. In the spraying process, thepowder mixture is sprayed onto the membrane via a nozzle, for example aslit nozzle. Generally, the membrane provided with a catalyst layer issubsequently heated to improve the bond between catalyst and membrane.The heating process can be performed via a hot roller, for example. Suchmethods and devices for applying the powder are described in DE 195 09748, DE 195 09 749 and DE 197 57 492, amongst others.

In the screening process, the catalyst powder is applied to the membraneby a vibrating screen. A device for applying a catalyst powder to amembrane is described in WO 00/26982. After applying the catalystpowder, the bond of catalyst and membrane can be improved by heatingand/or the step iv). In this connection, the membrane provided with atleast one catalyst layer can be heated to a temperature in the range offrom 50 to 200° C., in particular 100 to 180° C.

Furthermore, the catalyst layer can be applied by a process in which acoating containing a catalyst is applied to a support and the coatingcontaining a catalyst and present on the support is subsequentlytransferred to a polymer membrane. As an example, such a process isdescribed in WO 92/15121.

The support provided with a catalyst coating can be produced, forexample, by preparing a catalyst suspension described above. Thiscatalyst suspension is then applied to a backing film, for example madeof polytetrafluoroethylene. After applying the suspension, the volatilecomponents are removed.

The transfer of the coating containing a catalyst can be performed byhot pressing, amongst others. To this end, the composite comprising acatalyst layer and a membrane as well as a backing film is heated to atemperature in the range of from 50° C. to 200° C. and pressed togetherwith a pressure of 0.1 to 5 MPa. In general, a few seconds aresufficient to join the catalyst layer to the membrane. Preferably, thisperiod of time is in the range of from 1 second to 5 minutes, inparticular 5 seconds to 1 minute.

According to a particular embodiment of the present invention, thecatalyst layer has a thickness in the range of from 1 to 1000 μm, inparticular from 5 to 500, preferably from 10 to 300 μm. This valuerepresents a mean value which can be determined by averaging themeasurements of the layer thickness from photographs that can beobtained with a scanning electron microscope (SEM).

According to a particular embodiment of the present invention, themembrane provided with at least one catalyst layer comprises 0.01 to 20mg/cm², 0.1 to 10.0 mg/cm², preferably 0.3 to 6.0 mg/cm² andparticularly preferably 0.3 to 3.0 mg/cm² (sum of all precious metals).These values can be determined by elemental analysis of a flat specimen.

Furthermore, the membrane which can also be provided with a catalystlayer can further be cross-linked by action of heat in the presence ofoxygen. This curing of the membrane additionally improves the propertiesof the membrane. To this end, the membrane can be heated to atemperature of at least 150° C., preferably at least 200° C. andparticularly preferably at least 250° C. In this process step, theoxygen concentration usually is in the range of from 5 to 50% by volume,preferably 10 to 40% by volume; however, this should not constitute alimitation.

The cross-linking can also take place by action of IR or NIR(IR=infrared, i.e. light having a wavelength of more than 700 nm;NIR=near-IR, i.e. light having a wavelength in the range of from about700 to 2000 nm and an energy in the range of from about 0.6 to 1.75 eV,respectively). Another method is ß-ray irradiation. In this connection,the irradiation dose is from 5 and 200 kGy.

Depending on the degree of cross-linking desired, the duration of thecross-linking reaction can be within a wide range. Generally, thisreaction time is in the range of from 1 second to 10 hours, preferably 1minute to 1 hour; however, this should not constitute a limitation.

A membrane electrode unit according to the invention exhibits asurprisingly high power density. According to a particular embodiment,preferred membrane electrode units accomplish a current density of atleast 0.3 A/cm², preferably 0.4 A/cm², particularly preferably 0.5A/cm². This current density is measured in operation with pure hydrogenat the anode and air (approx. 20% by volume of oxygen, approx. 80% byvolume of nitrogen) at the cathode, with standard pressure (1013 mbarabsolute, with an open cell outlet) and a cell voltage of 0.6 V. In thisconnection, particularly high temperatures in the range of from 150-200°C., preferably 160-180° C., in particular 170° C. can be applied.

The power densities mentioned above can also be achieved with a lowstoichiometry of the fuel gases on both sides. According to a particularaspect of the present invention, the stoichiometry is less than or equalto 2, preferably less than or equal to 1.5, very particularly preferablyless than or equal to 1.2.

According to a particular embodiment of the present invention, thecatalyst layer has a low content of precious metals. The content ofprecious metals of a preferred catalyst layer which is comprised by amembrane according to the invention is preferably not more than 2mg/cm², in particular not more than 1 mg/cm², very particularlypreferably not more than 0.5 mg/cm². According to a particular aspect ofthe present invention, one side of a membrane exhibits a higher metalcontent than the opposite side of the membrane. Preference is given tothe metal content of the one side being at least twice as high as themetal content of the opposite side.

For further information on membrane electrode units, reference is madeto the technical literature, in particular the U.S. Pat. Nos. 4,191,618,4,212,714 and 4,333,805. The disclosure contained in the above-mentionedcitations [U.S. Pat. Nos. 4,191,618, 4,212,714 and 4,333,805] withrespect to the structure and production of membrane electrode units aswell as the electrodes, gas diffusion layers and catalysts to be chosenis also part of the description.

EXAMPLES

Table 1 shows the cell voltages of 4 different membrane electrode unitsat current densities of 0.2 A/cm² and 0.5 A/cm², respectively. Thevalues were recorded in a single fuel cell with an active area of 50 cm²at 160° C. Pure hydrogen served as the anode gas (with a stoichiometryof 1.2 and a pressure of 1 bara), air served as the cathode gas (with astoichiometry of 2 and a pressure of 1 bara). The composition of theindividual specimens is described below:

Specimen 1:

Anode A: The anode catalyst is Pt on a carbon support. The electrodeloading is 1 mg_(Pt)/cm².

Cathode B: The cathode catalyst is Pt on a carbon support. The electrodeloading is 1 mg_(Pt)/cm².

Membrane A: A polymer membrane doped with phosphoric acid, the polymerof which consists ofpoly-((2,2′-m-phenylene)-5,5′-bisbenzimidazole)-co-poly-((2,5-pyridine)-5,5′-bisbenzimidazole),serves as the membrane.

Specimen 2:

Anode A: The anode catalyst is Pt on a carbon support. The electrodeloading is 1 mg_(Pt)/cm².

Cathode B: The cathode catalyst is Pt on a carbon support. The electrodeloading is 1 mg_(Pt)/cm².

Membrane B: A polymer membrane doped with phosphoric acid, the polymerof which consists of poly-((2,2′-m-phenylene)-5,5′-bisbenzimidazole),serves as the membrane.

Specimen 3:

Anode A: The anode catalyst is Pt on a carbon support. The electrodeloading is 1 mg_(Pt)/cm².

Cathode B: The cathode catalyst is a PtNi alloy on a carbon support. Theelectrode loading is 1 mg_(Pt)/cm². The ratio of Pt to Ni is 1:1.

Membrane A: A polymer membrane doped with phosphoric acid, the polymerof which consists ofpoly-((2,2′-m-phenylene)-5,5′-bisbenzimidazole)-co-poly-((2,5-pyridine)-5,5′-bisbenzimidazole),serves as the membrane.

Specimen 4:

Anode A: The anode catalyst is Pt on a carbon support. The electrodeloading is 1 mg_(Pt)/cm².

Cathode B: The cathode catalyst is a PtNi alloy on a carbon support. Theelectrode loading is 1 mg_(Pt)/cm². The ratio of Pt to Ni is 1:1.

Membrane B: A polymer membrane doped with phosphoric acid, the polymerof which consists of poly-((2,2′-m-phenylene)-5,5′-bisbenzimidazole),serves as the membrane.

By comparing specimen 1 and 2, which include the same electrodes and Ptcatalysts but the differing membranes A and B, it can be seen that, byusing membrane B, an only very slight increase of the cell voltage by 3and 4 mV at 0.2 and 0.5 A/cm², respectively, can be achieved. If thecathode (specimen 3 with membrane A and specimen 4 with membrane B) ischanged to an alloy catalyst, as is described in this specification, theanode with Pt catalyst remaining the same, the cell voltage can beincreased substantially. In this way, the cell voltage increases by 22mV at 0.2 A/cm² with membrane A if said alloy catalyst is employed.Using membrane B, the cell voltage increases by 37 mV at 0.2 A/cm² if analloy catalyst is employed.

TABLE 1 Cell voltage in [V] at No. Cathode catalyst Membrane 0.2 A/cm²0.5 A/cm² 1 Pt/C, 1 mg_(Pt)/cm² A 0.634 0.555 2 Pt/C, 1 mg_(Pt)/cm² B0.637 0.559 3 PtNi/C, 1 mg_(metal)/cm² A 0.656 0.571 4 PtNi/C, 1mg_(metal)/cm² B 0.674 0.598

The invention claimed is:
 1. A membrane electrode unit consistingessentially of A) at least one polymer membrane which consistsessentially of at least one alkaline polyazole polymer with at least onenitrogen atom, the polymer membrane including at least one mineral acidwhich is phosphoric acid, B) at least two electrodes one which is acathode and one is an anode, wherein i) the cathode side consists of aPt/Ni catalyst the Pt/Ni catalyst being on a carbon support and ii) thecatalyst on the anode side consists of a Pt catalyst on a carbonsupport, iii) the alkaline polyazole polymer is a polybenzimidazole, iv)said catalyst on the anode side is loaded with 0.6 to 1 g/m² Pt based onthe surface area of the polymer membrane, v) said polymer membrane hasproton conductivity of at least 0.1 S/cm @ 120° C. and vi) the membraneelectrode unit providing for a current density of at least 0.3 A/cm³(cell voltage of 0.6) when air is used on the cathode side.
 2. Themembrane electrode unit according to claim 1, wherein a mixture of oneor more polybenzimidazole polymers with another polymer is employed. 3.The membrane electrode unit according to claim 1, wherein the polymermembrane comprises para-polybenzimidazoles.
 4. The membrane electrodeunit according to claim 1, wherein the catalyst is applied to thepolymer membrane.
 5. The membrane electrode unit according to claim 1,wherein the catalyst layer has a thickness in the range of from 0.1 to50 μm.
 6. The membrane electrode unit according to claim 1, wherein thecatalyst comprises catalytically active particles on a carbon support,the size of the catalyst particles being in the range of from 1 to 20nm.
 7. A fuel cell containing one or more membrane electrode unitsaccording to claim
 1. 8. The membrane electrode unit according to claim1, wherein the ratio of Pt to Ni is 1:100 to 100:1.
 9. The membraneelectrode unit according to claim 1, wherein said polymer membrane hasproton conductivity of at least 0.12 S/cm @ 120° C.
 10. The membraneelectrode unit according to claim 1, wherein the catalyst is on a carbonblack support, graphite support or graphitized carbon black support. 11.The membrane electrode unit according to claim 1, wherein the ratio ofPt to Ni is 1:100 to 100:1.
 12. The membrane electrode unit according toclaim 1, wherein the ratio of Pt to Ni is 1:1.
 13. The membraneelectrode unit according to claim 1, wherein the Pt/Ni is on the carboncatalyst and the Pt/Ni has a particle size of 1 to 20 nm and the carbonparticles have a particle size of 20 to 100 nm.
 14. The membraneelectrode unit according to claim 1, wherein the Pt/Ni is on the carboncatalyst and the Pt/Ni has a particle size of 2 to 6 nm and the carbonparticles have a particle size of 30 to 60 nm.
 15. The membraneelectrode unit according to claim 1, wherein the polymer membrane has athickness from 20 to 1500 m and the membrane is doped with phosphoricacid and the degree of doping expressed as mole per acid per repeat unitis from 15 to
 80. 16. The membrane electrode unit according to claim 1,wherein hydrogen gas is fed on the anode side.
 17. The membraneelectrode unit according to claim 1, wherein the membrane electrode unitproviding for a current density of at least 0.4 A/cm³ (cell voltage of0.6) when air is used on the cathode side.